Helicopters are normally powered by either a piston driven engine or a turbine engine. Power limitations for turbine engines are a result of several factors. Those factors typically include the following: an internal turbine temperature limit; a rotation per minute (RPM) limit; and a torque limit. Because a turbine engine can compress more air to increase the available air density, power limitations typically depend on the physical factors listed above.
For example, a turbine engine can not exceed its internal turbine temperature limit without damaging the engine. A turbine engine also cannot exceed its RPM limit without damaging the engine. The turbine engine can also not exceed its torque limit without damaging other components in the aircrafts system, such as a gearbox connecting the engine to the rotors. In effect, one of the above limits will usually act as a true limit for the turbine driven helicopter engine's power because one of the limits will normally be exceeded before the others. Various attempts to convey these limits to a pilot are shown in the prior art.
However, for a helicopter using a piston driven engine the power available at a given time depends on the ambient air pressure that is available for mixing with engine fuel. The ambient air pressure varies as a function of the actual altitude of the helicopter and the ambient air temperature at that altitude. The actual altitude and ambient air temperature can be used to calculate a density altitude or equivalent altitude. Typically, the ambient air pressure will decrease by one inch of mercury per 1,000 feet of pressure altitude. For example, at an actual altitude of 2,800 feet with an ambient air temperature of 38 degrees centigrade, the density altitude would be approximately 6,186 feet, and the ambient air pressure would be approximately 27.2 inches of mercury.
At sea level the available atmospheric pressure is measured to be approximately 29.92 inches of mercury and will normally decrease by one inch of mercury for each additional 1000 feet of pressure altitude. At sea level, the atmospheric pressure results in a maximum power available to a helicopter employing a piston engine, i.e. 29.92 inches of mercury of available manifold pressure. However, the typical piston engine will have a manufacturer determined maximum pressure that is lower than that value. For example, a Robinson R44 helicopter engine, model O-540-F1B5 manufactured by Lycoming of Williamsport, Pa., typically has a maximum allowable manifold pressure of 26.3 inches of mercury, while an R22 helicopter engine model O-360 also manufactured by Lycoming has a maximum allowable manifold pressure of 25.9 inches of mercury. In this context, the maximum pressure is typically measured by a sensor attached to a manifold of the piston engine and is referred to as manifold pressure (MP). As engines have improved, and “de-rating” has become popular, the maximum manifold pressure limit has remained relatively constant or has even been reduced thus allowing aircraft manufacturers to increase the altitude up to which the maximum engine power will still be available. The highest altitude at which a piston aircraft engine can produce its maximum allowable power is called the “critical altitude.” Above the critical altitude the ambient air pressure is insufficient for a normally aspirated (non-turbocharged) piston driven aircraft engine to generate the maximum power, as set by its manufacturer.
Typically, the manufacturer will set three power limits, as follows: a maximum continuous power; a five-minute maximum power; and an absolute maximum power. A helicopter requires the most power at take off and when hovering, and these activities will often require that the engine be operated in the five-minute maximum power range.
Existing attempts to show a pilot the above-described limits (e.g., colored arcs painted on the face of a Manifold Pressure (MP) gauge) are only valid for one set of predetermined external conditions (i.e. a selected altitude and a selected temperature). Static indicia of performance limits are only valid at those specific conditions. Given the broad range of altitude and temperatures that can be encountered by a piston powered aircraft, the colored arcs often provide false confidence that more power is available than current conditions dictate, or the colored arcs depict overly conservative usable power limits.
Known instruments provide no indication to the helicopter pilot of the maximum amount of power actually available at a given altitude and temperature. Manufacturers usually do not even provide a way to calculate how much power is actually available. Where the actual power available exceeds the existing indication of limits (e.g., the green colored arc painted on the manifold pressure gauge) there is no special problem; the pilot merely needs to be sure not to exceed the maximum power limit set by the manufacturer. However, safety problems can occur when the actual power available is less than the maximum power limitation because the pilot may be unaware that the aircraft can not produce the maximum power limitation. Below the critical altitude there is always more power available than the manufacturer set maximum power limit. However, above the critical altitude the power available will be below the manufacturers limit. A pilot flying above the critical altitude currently has no way of knowing how much power is actually available, except to understand that the maximum available will be less than the manufacturer maximum limit.
In the case where a pilot attempts to use power in excess of what is actually available, potential safety of flight issues arise. A helicopter engine in flight generally operates at a fixed RPM (e.g., 100%) and its power demand is varied by a “collective control.” At a given set of conditions (altitude, temperature and collective pitch), a pilot has no good way of knowing whether further increases of collective pitch (hence asking the engine for more power) would exceed the MP (power) that is actually available. Exceeding available engine power causes reduced rotor RPM, which if not corrected by immediate pilot action results in “rotor stall” with often catastrophic results. Rotor stall often occurs in conjunction with high altitude takeoffs and hovering in flight.
The need remains for an indicator that determines and conveys to a pilot a reliable indication of the actual power available during flight, especially when the aircraft is above its critical altitude.